In recent years, a reflection-preventing film is provided on a lens surface of an imaging lens of, for example, a camera or the like in order to prevent reflection of unnecessary light, such as a ghost or a flare.
As the reflection-preventing film provided on the surface of an optical element, such as the lens, for example, there is known a multilayer thin film on which a high refractive index layer and a low refractive index layer are alternately and appropriately superimposed on each other according to the wavelength of light at which reflection is prevented. Although such a multilayer reflection-preventing film is formed by a vacuum process, such as vacuum evaporation or sputtering, since the multilayer reflection-preventing film needs to provide the multi-layer thin film of which the film thickness is managed, the treatment time becomes long. Additionally, since the directivity is high in the vacuum process, the film thickness varies depending on the lens shape at a central portion and an outer peripheral portion, for example, in the case of a lens in which the amount of irregularity of the outer peripheral portion with respect to the central portion is higher. For this reason, a reflection-preventing film that has uniform reflection preventing characteristics over the entire lens surface cannot be obtained, for example, a reflection-preventing film in which the reflection preventing characteristics of the outer peripheral portion are inferior to the central portion is obtained.
As a reflection preventing structure that is not based on such a multilayer thin film, as described in Japanese Unexamined Patent Application, First Publication No. 2006-317807, for example, there is known a reflection preventing structure in which a microstructure is formed in units of a triangular pyramid, a quadrangular pyramid, or the like on a lens surface so that refractive index variation occurs in the vicinity of the lens surface.
In Japanese Unexamined Patent Application, First Publication No. 2006-317807, an X-ray mask in which triangular prisms are arrayed at a minute pitch is formed, and a resist coated on a lens via this X-ray mask is exposed to X rays to form a triangular pyramidal microstructure on a lens surface, and RF dry etching is further performed to transfer a triangular pyramidal microstructure to a lens surface to form a master having a reflection preventing structure on the surface thereof. Next, a technique of transferring the shape of this master to manufacture a Ni duplicating mold through an electroforming process and of manufacturing a lens having a reflection preventing structure on the surface thereof through molding using this Ni duplicating mold is described.
Additionally, in Japanese Unexamined Patent Application, First Publication No. 2004-12856, a technique of molding a reflection preventing part in which fine conical convex portions are arranged substantially densely is described. In the technique in Japanese Unexamined Patent Application, First Publication No. 2004-12856, a thin layer of an etching rate gradient material is formed on a molding surface of a forming mold made of a glass plate, a photoresist film is formed on the surface of this thin layer, exposure and development are performed on this photoresist film, to form a mask with a predetermined pattern, and the etching rate gradient material layer is etched via this mask. Accordingly, a forming mold that transfers the shape of the reflection preventing part is formed. Then, by performing press molding using this forming mold, an optical element in which the shape of the reflection preventing part is transferred to the lens surface is formed.